Direction finding is the measurement of direction of incoming signals received by a direction finding antenna unit, for instance radio or other electromagnetic waves.
Generally, the direction finding antenna unit comprises an antenna system which has several antenna elements for receiving the incoming signals. For instance, the several antenna elements are formed in a circular array. It is known that in certain frequency ranges, the antenna elements have undirected directional characteristics (also called non directional characteristics) when they are considered in isolation. In order to address this problem, one antenna element of the several antenna elements is used as a reference antenna element. Then, the phase difference and the orientation, in particular azimuth and elevation, are related to this reference antenna element during direction finding. However, the receiving characteristics of the whole antenna system may be restricted due to the directional characteristics of the antenna elements in certain frequency ranges. For instance, the antenna beam of the reference antenna element is disturbed by antenna beams of the other antenna elements. Thus, the level of the incoming signals and the signal-to-noise ratio heavily depend on the incident angle as well as the frequency of the incoming signals. In worst case, weak signals cannot be detected and processed by the direction finding antenna unit due to the restricted receiving characteristics of the whole antenna system.
In the state of the art, this problem is addressed by a separate antenna element used as the reference antenna element which is disposed in a certain distance to the antenna system such that the antenna elements of the antenna system do not disturb the receiving characteristics of the reference antenna element. However, the size of such a direction finding antenna unit is large due to the separate reference antenna element.
Despite this, it is known in the prior art to select several antennas of an antenna system due to shadowing effects while only taking the signal strength of the incoming signal at each antenna into account. For instance, DE 10 2013 226 988 A1 shows such a system.
FR 2 936 382 A1 discloses a method for identifying several emitters wherein the signals received are grouped accordingly.
U.S. Pat. No. 4,313,117 describes a system having several fixed antenna pairs wherein a control unit selects the antenna pairs receiving the signals with the highest signal strength for further processing.